In general, a heat insulating container forming a battery assembly container of a sodium-sulfur battery (hereinafter referred to as the NaS battery) as a secondary battery operated at high temperature is made up of a container body having an opening opened at is upper position and a lid body fitted to the opening. A battery assembly formed by connecting a large number of battery cells in a predetermined arrangement pattern is accommodated in this heat insulating container. Electrodes pass through one end and the other end of the heat insulating container, and the electrodes are connected to both ends of the battery assembly.
Since this NaS battery is operated at about 300° C., it is required to heat the NaS battery up to this temperature. Further, it is required to maintain the temperature of the NaS battery at the operating temperature for charging and discharging of the battery. Moreover, it is required to ensure the uniform temperature distribution in the heat insulating container so that the battery cells exhibit the intended characteristics sufficiently. To this end, an electric heater is provided, e.g., at the bottom inside the heat insulating container. By this heater, the temperature in the heat insulating container is regulated (see Japanese Laid-Open Patent Publication No. 06-283215 and Japanese Laid-Open Patent Publication No. 08-078051).